Photographic materials containing silver halide are sensitized to various regions of the spectrum through the use of sensitizing dyes. In many situations, such as with graphic arts materials, it is desirable to provide a red sensitive silver halide photographic material that can be exposed with, for example, a helium neon laser. It is also desirable that such a material be handleable under a safelight, e.g., a green safelight. Therefore, the sensitizing dye should impart sensitivity to silver halide in the red region of the spectrum, but not in the green region.
Classes of sensitising dyes have previously been proposed for preparing silver halide materials that are both sensitive to the red radiation produced by a Helium Neon laser and relatively non sensitive to safelight illumination. For example, British Patent Specification 1,471,701 describes the use of certain tri heterocyclic dyes for this purpose. A Problem with such dyes, however, is that they leave a residual stain in the processed material.